While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, DUV and EUV lithography processes are thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using an ArF excimer laser as the light source is requisite to the micropatterning technique capable of achieving a feature size of 0.13 μm or less.
The ArF lithography started partial use from the fabrication of 130-nm node devices and became the main lithography since 90-nm node devices. For the next 45-nm node devices which required an advancement to reduce the wavelength of exposure light, the F2 lithography of 157 nm wavelength became a candidate. However, for the reasons that the projection lens uses a large amount of expensive CaF2 single crystal, the scanner thus becomes expensive, hard pellicles are introduced due to the extremely low durability of soft pellicles, the optical system must be accordingly altered, and the etch resistance of resist is low; the F2 lithography was postponed and instead, the early introduction of ArF immersion lithography was advocated. This enables mass-scale production of 45-nm node devices. For the mass-scale production of 32-nm node devices, the double patterning process utilizing sidewall spacer technology is used although the process suffers from complexity and length.
For the fabrication of 32-nm node and subsequent devices, the EUV lithography using an exposure wavelength of 13.5 nm which is shorter than the conventional lasers by one order of magnitude and thus featuring improved resolution is expected rather than the double patterning process with noticeable costs. Efforts are focused on the EUV lithography.
In the EUV lithography, a low laser power and light attenuation by reflecting mirror lead to a reduced quantity of light. Then light with a low intensity reaches the wafer surface. It is urgently demanded to develop a high-sensitivity resist material in order to gain a throughput despite a low light quantity. However, a trade-off relationship of sensitivity is pointed out that the sensitivity of resist material can be increased at the sacrifice of resolution and edge roughness (LER, LWR).
As the circuit line width is reduced by the recent rapid advance of technology, the degradation of contrast by acid diffusion becomes more serious for the resist material. This is because the pattern feature size is approaching the diffusion length of acid. Acid diffusion leads to degradations of mask fidelity and pattern rectangularity and non-uniformity of a fine line pattern, i.e., line width roughness (LWR). Accordingly, to gain more benefits from a reduction of exposure light wavelength and an increase of lens NA, an increase in dissolution contrast and suppression of acid diffusion are required more than in the prior art resist materials.
One approach to overcome these problems is to bind a PAG in a polymer. For instance, aiming to improve sensitivity, Patent Document 1 proposes a polymer using an acryloyloxyphenyldiphenylsulfonium salt as a monomer. Patent Document 2 proposes to incorporate the monomer into a polyhydroxystyrene resin for improving the LWR of this base resin. However, since the sulfonium salt is bound at its cation side to the polymer, the sulfonic acid generated therefrom upon exposure to high-energy radiation is equivalent to the sulfonic acids generated by conventional PAGs. These proposals are thus unsatisfactory to overcome the outstanding problems. Also, aiming to improve sensitivity and resist pattern profile, Patent Document 3 discloses sulfonium salts having an anion side incorporated into the polymer backbone such as polystyrenesulfonic acid. The acids generated therefrom are arenesulfonic and alkylsulfonic acid derivatives which have too low an acid strength to sever acid labile groups, especially acid labile groups in acrylate-derived base resins. The acrylate resins are commonly used not only in the ArF chemically amplified lithography offering a fine feature size, but also in the EB and EUV lithography processes. Also a variety of anion-bound resins capable of generating an acid having high acid strength have been developed. Patent Document 4 discloses a polymer having a difluoroethanesulfonic acid anion in the backbone. Patent Documents 5 and 6 disclose a polymerizable sulfonium salt having a partially fluorinated sulfonic acid anion and a resin obtained therefrom. Acid diffusion is suppressed by incorporating a strong acid-generating anion in the backbone of a base resin. Although some improvements are made in resist properties including mask fidelity, pattern rectangularity and LWR, they are still unsatisfactory.
In EB or EUV lithography which is regarded promising as the exposure technology following ArF lithography, exposure must be done in vacuum (or reduced pressure). Thus sulfonic acid generated during exposure will volatilize. This gives rise to the problems that no satisfactory pattern profile is obtainable, and that volatile sulfonic acid or volatile decomposed products of sulfonium cation (e.g., phenylsulfides) form bubbles (known as outgassing), causing damages to the exposure tool. With respect to the suppression of outgassing from acid generators, various approaches are taken as disclosed in Patent Document 7 and the like, but none of these approaches are satisfactory.
In the EUV laser source of the laser-produced plasma (LPP) method wherein CO2 laser light is irradiated to tin particles to emit EUV of wavelength 13.5 nm, weak light of longer wavelength 140 to 300 nm is emitted besides the desired EUV. This longer wavelength light is known as out-of-band (OOB) light. Since OOB floods over the entire surface, the resist film exposed to OOB is reduced in contrast and experiences a film thickness loss in the otherwise unexposed region. The EUV microstepper is loaded with a Zr filter as a means for cutting off OOB light, but the quantity of light is reduced thereby. The EUV scanner may not be loaded with the filter because a reduction of light quantity is not permissible for the goal of enhancing the throughput. In the EUV lithography, there is a need for a resist material which is highly sensitive to EUV, but less sensitive to OOB.
For the development of such resist materials, the cation structure of sulfonium salt PAG is important. Patent Document 8 (JP-A 2011-138107, paragraph [0052]) describes a polymer-bound acid generator having a high sensitivity to EUV light, but a low sensitivity to OOB light. Since its lithographic characteristics are still unsatisfactory, it is desired to have a resist material having a lower OOB sensitivity. Non-Patent Document 1 describes the superiority of a protective film which is formed on top of the resist layer for cutting off OOB light.